1. The Field of the Invention
The present invention refers to a fluorine- and lead-free and preferably arsenic-free optical phosphate glass, the use of such a glass in the areas of imaging, projection, telecommunication, optical telecommunication engineering, mobile drive and laser technology as well as optical components or pre-forms of such optical components.
2. The Description of the Related Art
In recent years the trend of the market in optical as well as opto-electronical technologies (areas of application imaging, projection, telecommunication, optical telecommunication engineering, mobile drive and laser technology) increasingly goes in the direction of miniaturization. This can be seen from the increasingly smaller end products and requires cumulative miniaturization of single parts and components of such end products. For manufacturers of optical glasses this development is connected to an articulately decreasing volume of demanded raw glass despite higher numbers of end products. At the same time, increasing pressure of prices towards the glass manufacturers arises on the part of subsequent processing industry, because there is far more waste in relation to the product when manufacturing smaller components from block or ingot glass and the processing of such small parts is requiring much more effort than with bigger components.
Thus, instead of detaching glass portions for optical components from block or ingot glass as it was usual up to now, recently processing techniques gain importance that, directly after the glass melt, provide for near-net-shape pre-forms like for example gobs or spheres. For example the demand of the subsequent processing industry for near-net-shape pre-forms for re-pressing, so called “precision gobs”. “Precision gobs” are preferably understood as completely fire-glazed, semi-free or free formed glass portions that are already portioned and of a geometry that is near the final geometry of the optical component.
Such precision gobs can advantageously be converted into optical elements like lenses, aspheres etc. by so called precise pressing or precision moulding. A further processing of the geometrical form or of the surface by for example surface polishing is not necessary anymore. Using this method allows to flexibly provide the required small glass melt volumes (distributed into large numbers of small pieces of material) with small changeover times. Because of the relatively low number of cycles and pieces and usually small geometries, the added value of the method cannot be derived from the value of the material alone. Hence, the products must leave the press in a state that is ready to system installation, i.e. one must be able to abstain from time-consuming finishing, cooling and/or cold post-processing steps. For such pressing methods precise instruments with high-value and, hence, expensive form materials must be used, because of the required high geometrical precision. The cost-effectiveness of the produced products and/or materials is thus strongly influenced by stand-still periods of the forms. A very important factor of high stand-still periods is a preferably low operating temperature which can however only be lowered as long as the viscosity of the materials to be formed is sufficient for the pressing process. There is, hence, a causal relationship between the processing temperature and thereby the glass transition temperature Tg of a glass to be processed and the cost-effectiveness of such a moulding process: the lower the glass transition temperature of a glass, the higher are the stand-still periods of the forms and the higher is the profit margin. From this context it can be derived that there is a need for so-called “low-Tg glasses”, that means glasses with low melt and transition points, i.e. glasses that show a viscosity that is suitable for processing at preferably low temperatures.
In the process engineering of the melt there has newly been an increasing demand for “short” glasses, i.e. glasses strongly changing viscosity—in a certain viscosity range—with a relatively small variation of temperature. This behaviour bears the advantage during the melting process that the hot-pressing times, i.e. the tight-fit times can be decreased. With this improvement the throughput is increased, i.e. the clock cycle is reduced, on the one hand. And on the other hand the mould material is preserved which—as described above—has positive effects on the overall production costs. Such “short” glasses have as a further advantage that with quicker cooling down than “longer” glasses, also glasses with higher crystallisation tendencies may be processed. A preformation of seed crystals that could pose problems in secondary moulding processes is avoided. This offers the possibility to draw fibres from such glasses.
Furthermore it is desirable that the glasses, next to the above-mentioned properties and the necessary optical properties, are sufficiently chemically resistant and have preferably small extension coefficients.
In the state of the art glasses with similar optical positions or comparable chemical compositions are described, but these glasses suffer from considerable drawbacks. Especially, many of these glasses contain higher amounts of fluorine and/or Li2O that evaporate very easily during melting processes which renders a precise adjustment of the glass composition difficult. This evaporation also has negative effects on the pressing process wherein the glass is heated up again and could deposit on the surface of the moulds and form precipitations on the glass. Furthermore many glasses contain SiO2 that as a network former—increases the glass transition temperature of the glass and causes a longer viscosity curve.
Higher amounts of the component Nb2O5 (more than 22% by weight) and of the component TiO2, too, are described in the state of the art as obligatory components. Nb2O5 as well as TiO2 strongly increase the refractive index (nd) and as strongly decrease the Abbe number (vd). In order to achieve the optical position of nd 1.60 to 1.64 and vd of 56 to 64 the content of Nb2O5 and/or TiO2 should be very small, especially smaller 1% by weight or preferably these components should not be contained in the composition.
JP 2001058845, JP 2003300751 and JP 08104537 describe optical glasses with high refractive indices and high dispersions partly for precision moulding. These glasses imperatively comprise the component Nb2O5 in very high amounts. Therewith an Abbe number >56 cannot be provided.
US 2005/054511 describes an optical glass for precision moulding that imperatively comprises the colouring component CuO.
DE 1023861 refers to an optical glass comprising alkali earth metals of 10-40% by weight (see claim 1). In order to allow for a high refractive index and a high Abbe number however alkali earth metal contents of more than 40% are needed. Furthermore the glasses according to this document do not comprise aluminium oxide. This component however is obligatory in order to improve the chemical resistance of phosphate glasses. The same applies to the desired steepness of the viscosity curve in the defined range. Finally the environmentally unsafe cadmium oxide as well as in part lead oxide is part of the glasses disclosed in DE 1023861.
DE 1089934 describes an optical crown glass with a refractive index of 1.53 to 1.68 in which the sum of the contained alkali, alkali earth, aluminium, zinc, cadmium, lanthanum, arsenic, antimony, lead and bismuth oxides is not more than 48% by weight. Such glass systems do not provide for a balance between the refractive index on the one hand and the Abbe number on the other hand as achieved by the present invention.
DE 1421879 also describes a crown glass with an Al2O3 content of less than 5% by weight and/or a content of La2O3 of more than 2% by weight. A aluminium oxide content of less than 5% by weight causes a bad chemical resistance. The high content of lanthanum oxide leads to an increase of refractive index, however simultaneously leads to a strong decrease of the Abbe number to values below 56. Furthermore the glass transition temperature of the glass is undesirably raised.
JP 2001064036 comprises a phosphate glass as filling material in resins. Glasses with a high P2O5-content of 40 to 90 mol % are claimed. With such a high content of P2O5 a refractive index of >1.6 cannot be achieved.
DE 19826637 describes an optical glass with photoelastic coefficients and imperatively contains the toxic component PbO.
JP 61040839 comprises an optical phosphate glass with a refractive index of 1.52 to 1.89 and an Abbe number of 26 to 65. The glass imperatively comprises the component Sb2O3 in an amount of at least 1% by weight. Sb2O3 in this case serves as a glass forming agent and may have negative effects on high transmission in these high contents. In the glass according to the present invention Sb2O3 may be present merely as a fining agent in an amount of up to 0.5% by weight.
The document U.S. Pat. No. 6,127,297 describes optical glasses with a specific density of <3. The essential component is further to P2O5, TiO2 present in an amount of at least 1.15% by weight, displacing the UV edge of the glass to the longer wavelength region.
U.S. Pat. No. 4,771,020 describes an optical phosphate glass comprising proportions of Sb2O3, Bi2O3 and of toxic PbO in a range of from 34 to 74% by weight.
US 2004/0259714 comprises an optical glass for precise pressing with a refractive index of from 1.55 to 1.71 and an Abbe number of from 57 to 70. The glass obligatorily contains P2O5 in relatively high amounts of from 28 to 50 mol %. Furthermore high contents of alkali oxides are contained, leading to undesirable decrease in refractive index.
JP 11199269 describes an optical BaO containing phosphate glass with a low photoelastic constant and a low amount of B2O3, namely up to 4% by weight.
In U.S. Pat. No. 3,278,318 a glass is disclosed that comprises very high proportions of WO3 with 15 to 85% by weight, not providing for Abbe numbers of ≦56.
US 2005/0113239 discloses a glass having extraordinarily high contents of Nb2O5 of 35 to 65% by weight. This does not allow for high Abbe values.
The glasses disclosed in DE 1596854 contain high amounts of P2O5. and are basically fluorine containing glasses (see claim 2, specifying claim 1, saying that high amounts of fluorides are present). Furthermore the glasses may contain undesired components like lead or thorium.
US 2006/0150682 A1 describes glasses with an Abbe number in a range between 59 to 70, all of which containing the very volatile component Li2O in an amount of up to 20% by weight.